The present invention is concerned with drive mechanisms employed in article handling apparatus in which an article or part is located upon a support in a stationary ready position in the path of movement of a retracted part transfer member. The part transfer member is then driven from its retracted position in a working stroke in which the transfer member moves from its retracted position into engagement with the part and lifts the part from its support to an elevated position clear of the support.
One example of such an apparatus is a lift and carry conveyor in which parts are advanced in step-by-step movement along a fixed support by a carrier which is cyclically actuated in a four step cycle in which during a lifting stroke the carrier lifts the parts clear from the fixed support, then advances the parts forwardly one step and then lowers the parts back onto the support. The carrier is then returned beneath the parts to its original start position.
In another type of apparatus, the transfer member or carrier may move upwardly beneath a part temporarily located at a ready position in a transfer machine and lift the part from its ready position into operative alignment with a tool or part receiving clamp.
The apparatuses generally described above require the carrier, when in its stationary start position, be spaced below the part when the part is located in its ready position so that the part can move into its ready position without interference from the carrier. The drive which drives the carrier in its lifting stroke must be arranged so that the carrier is smoothly decelerated to a stop as it reaches the upper end of its lifting stroke.
Typically, such drives employ a crank mounted for rotary oscillation about a horizontal axis with a roller at the distal end of the crank received within a horizontal slot in the carrier. Typically, the crank is so oriented that the slot engaging roller of the crank is at its twelve o'clock position relative to the crank axis when the carrier is at the upper end of its stroke, this arrangement providing a smooth decrease in the upward velocity of the carrier to a zero upward velocity as the carrier arrives at the upper end limit of its stroke. Assuming that rotation of the crank is continuous through its lifting stroke cycle, this means that the carrier will be moving upwardly with a positive upward velocity at the moment it contacts the stationary part, typically at or close to the mid point of its upward stroke. Because the crank radius is fixed, the upward velocity of the carrier at the instant it contacts the part is directly dependent upon the speed of the motor employed to drive the crank in rotation at that instant. If, in the simplest approach, the drive motor drives at a substantially constant speed at least during the midpoint of its cycle, this constant speed must be limited to one which will not result in an unduly high impact between the carrier and part. If, on the other hand, the motor is slowed to a reduced speed as the carrier approaches the part, the total cycle time will obviously be substantially increased unless the control accelerates the motor after it contacts the part. In many arrangements, the geometry of the system is such that the mechanical advantage of the crank is substantially reduced or at a minimum at the instant it contacts the part, which will impose a substantial load on the drive motor if the drive motor is required to accelerate the carrier at this point.
The present invention is especially directed to a "soft touch" drive mechanism in which a drive motor which drives at a substantially constant speed through a given drive cycle will drive a carrier member in a working stroke in which the velocity of the carrier is momentarily reduced to zero at the moment it contacts the part--i.e. the carrier engages the part with an impactless "soft touch"--and in which the drive mechanism operates at a maximum mechanical advantage during those portions of the cycle when the carrier is moving at minimum speed. The mechanism is so designed that engagement between the carrier and part need not occur at the exact midpoint of the carrier stroke.